The invention relates to a method processing video signals having the form of digitally encoded samples and being arranged so that successive samples produce a television picture on a display screen by line and field sequential scanning, the method comprising the steps of:
(a) receiving from a video signal source an input video signal comprising successive video signal samples representing an input television picture; PA0 (b) applying to said input television picture a given geometrical modification by: PA0 (b1) determining on the basis of said given geometrical modification a set of addresses for addressing memory locations of a video frame store; PA0 (b2) assigning to each sample of the input video signal or to samples derived therefrom a memory location indicated by an address of said set of addresses; PA0 (c) reading video signal samples from at least a selected part of said video frame store. PA0 (d) determining whether there were memory locations of said selected part to which no sample has been assigned; PA0 (e) generating a control signal when a memory location to which no sample has been assigned is detected; and PA0 (f) inserting under control of said control signal, a substitute sample generated from at least one adjacently located sample, at a location in the successive samples read out from the store, which location corresponds to said memory location to which no sample has been assigned.
The invention also relates to apparatus for processing video signals having the form of digitally encoded samples and being arranged so that successive samples produce a television picture on a display screen by line and field sequential scanning, the apparatus comprising a frame store, a first address generator for generating a set of addresses for addressing memory locations in the frame store the set of addresses being generated from input signals defining a given geometrical modification of the input television picture represented by input video signal samples, said first address generator being effective to assign to each sample of input video signal or samples derived therefrom a memory location indicated by an address of said set of addresses, and a second address generator for generating addresses of at least a selected part of the frame store to enable video signal samples to be read successively from each location of the selected part of the frame store to form a series of video signal samples which represent the modified television picture.
Such a method and apparatus have been disclosed in UK Patent Application No. 2119197A which describes how the method and apparatus may be used to store a television picture in a field store at a selected rotational angle. In television broadcasting there is often a requirement for studio equipment which is capable of rotating a video image for producing special effects. The field store stores picture element samples in an analog of the television picture and is arranged to be read on a line and field sequential scanning basis. The rotation of the picture is achieved by suitably modifying the input addressing to write the input video signals into selected locations in the store so that a rotated image is stored. One problem in such a system is that at certain angles of rotation there is insufficient input information to fill every location in the store. Thus when the store is read out holes appear in the displayed picture, that is certain picture elements are missing because there was no sample information stored in a particular store location. The same problem will occur with other modifications of the input television picture which involve expansion of part of the picture when the manipulation is effected by modifying the input addresses applied to the store.